Wave signaling system



Dec. 18, 1928.

J. F. DREYER, JR

WAVE SIGNALING SYSTEM 4 Sheets-Sheet 1 Filed Feb. 23, 1927 INVENTOR Jb/m l Dre ye Jr.

ATTORN EY$ Dec. 18, 1928. 1,695,842

J. F. DREYER, -JR I WAVE SIGNALING SYSTEM Filed Feb. 23, 1927 I 4 Sheets-Sheet 2 ATTORNEYS Dec. I8, 1928.

. 1,695,842 J. F.'D REYER, JR

WAVE SIGNALING SYSTEM Filed Feb. 23, 192'? -4 Sheets-Sheet 3 INVENTOR MEQLENG 11v KILO crcLEJ. J0 [m F2761"! 5 'Dec- 18: 19281 AMPLIFICATI0NI N E 8 N 3.

1,695,842 J. F. DREYER, JR

WAVE SIGNALING SYSTEM Filed Feb. 23, .1927

VOL T4 GEAMPIIIHA Tia/v PER STA GE FOHHIGH-FREOl/ENC)TRANSFORMERS.

100 200 300 400 500 600 WAVE LENGTH (1 15mm) INVENTOR ATTORNEYI';

, 4Sheets-Shet 4 Patented Dec. 18, .1928.

UNITED STATES PATENT oer-" ce.

JOHN F. 'DREYER, an, or nnooxmm, NEW YORK, ASSIGNOR To HAZELTINE conronA- TION, A conronA'rIoN or DELAWARE.

WAVE STGNALINQ SYSTEM.

Application filed February 23, 1927. SeriarNo. 170,179.

oscillate because of undesired capacitive and la'magnetic couplings between portions of the respective input and output circuits. When but one or two stages of high-frequency ama 'plifi'cation are employed, this tendency toward oscillations can be minimized or elim-.

inated by various expedients; but the necessity for the provision of greatly increased high-frequency amplification, as by employing more than two highly efficient ampIifica-- tion stages, has presented several distinctly'new problems, of which a satisfactory solution is realized in the present invention. Radio receivers including a multista e highfrequency vacuum'tube amplifier embodying the improvements introduced by the present invention, and of the t pe hereinafter described, will afford the fo lowing advantages:

(1) complete freedom from undesirable oscil lations and signal distortion resulting .in

. faithful amplification; (2) a high degree of amplification, or gain, per stage; (3) freedom from signal pick-up by the component electrical units or elements and wiring, except through the antenna or antenna. input circuit; (4) substantiall 'uniform high-frequency amplification t roughout the entire operating frequency range of the system; not-necessarily more than two manual tuning controls, regardless of the number of tuned circuits; (6) good selectivity Without .critical tuning adjustments; (7 complete vohune control independent of tuning"adjust-ment andwithout sacrificeof fidelity of amplification; and (8) convenient physical size, and arrangement of components whereby the overall dimensions of thecabinet are minimized.

These advantages, while of independent merit, are, in general, nevertheless, the resultv oimutually dependent and correlated means cooperating to efiect a substantialimprove ment, as will be observed from a consideration of the following description of a preferred embodiment of this invention illustrated in the accompanying drawings, in which Fig. 1 is .a front-of-panel view of a complete radio receiver to which the several features'of the present invention have been applied;

Fig. 2 shows a top view of the same receiver, with the cover removed, and illusci pal component parts;

Fig. 3 is a diagrammatic representation of the circuit arrangement -of the radiofrequency stages and detector shown in Figs. 1 and 2;

Fig. 4 represents graphically typical resonance curves, ortuning characteristics, of the sharply tuned, and of the broadly tuned high-frequency amplifying stages;

Fig. 5 represents graphically the voltage amplification per stage of. high-frequency transformers, a) as employed previous to the present invention, and (b) in accordance with the present invention.

, Referring especially to Figs. 1 and 2, there are illustrated two views of a radio receiver completely enclosed within a cabinet of convenient proportions. This cabinet contains a metal box divided by metallic partitions 5, into five separate compartments numbered from left to right (1) to (5), inclusive, which in effect are five compartments electrically bonded together.

Each of the first three compartments completely encloses an individual high-frequency amplification stage, and because each compartment resembles a box, it is herein termed a box shield. The joints of each compartment or box shield should be soldered, welded, or crimped mechanically, in order to eliminate all possibility of leakage of magnetic. flux. The top-of each box shield has anopening 50 therein, through which a vacu um tube such as V may readily be inserted orremoved from the vacuum tube socket- VS within the compartment. The vacuum tubes trates the 'physical arrangement of the prinleft may partially protrude through the o n .ings 50 so that they maybe easily graspe for removal from the receiver. Metal covers 51 close the openings against stray-couplings.

In Fig. 2, the first com artment, or box shield on the left, numbere (1) is indicated as containing I '2 V i in] T a'n adjustable tuning condenser C and a vacuum tube socket VS,. These three components, together with a suitable vacuum tube V may be considered to comprise the first tuned stage. The compartments numbered (2), (3) and (4) are substantially identical with each other, and, similarly, they each contain a tuning condenser C5, C and C respectively, an interstage transformer T T and T respectively. and a vacuum tube socket VS,, VS and VS,. Compartment (5), at the extreme right, contains lowor audio-frequency apparatus and therefore need not be completely shielded. Compartment (5) is indicated as including a vacuum tube socket VS, for the first low-frequency amplifying tube, and a second socket VS for the second low-frequency, amplifying tube. The first low-frequency tube V, is intended to be coupled to the output circuit of the detector tube V, by means of low-frequency transformer T Likewise, the second low-frequency tube V, is coupled to the first low-frequency tube by means of transformer T It will benoted that the first tuning condenser C, is fastened directly upon the control panel 24, and is independently adjustable by a manual control 21. The three additional tuning condensers, C C and C,i,'.; were, in theactual receiver illustrated, mechanically secured to the frameof the containing cabinet, although, of course, any other convenient means of support could be provided. These three condensers are advantageously arranged with their rotatable shafts 19 parallel to the plane of the control panel, and with their ends abutting and coupled together with a suitable .metal coupling, or alternatively, the rotors of condensers C 0, and C, may be mounted on, a single metal rod or shaft. These condenser rotors are thus electrically connected together through theshaft 19 common'to the three condensers. The shaft is rotated through a right-angle mitre gear 18 by manual control knob 20. By means of this arrangement all tuning condensers of the stages other than the firstmay be. adjusted simultaneously by tuned high-frequency. stages. invention introduces this simplicity of tunthe single-control knob 20. The arrangement described thus limits the number of tuning controls to two, regardless of the number of The present ing controls by reason of the provision that the first, or antenna, stage shall be sharply tuned, and the subsequent high-frequency stages shall be broadly tuned with respect to the first stage.

It is well recognized that the coupling of several high-frequency tuning condensers forsimultaneous control by a single adjusting device introduces many manufacturing difficulties, one of the most important of which is the requirement that a. given annular change of the oontrol'shaft shall effect an exactly equal capacity change in all of the coupled condensers. When it is required that all of the high-frequency amplification stages have a sharp resonance characteristic, and be controlled by a single adjusting device, the

.tuning condenser individual to each stage must necessarily be manufactured with extreme precision.- Experience has shown that the cost of manufacturing such condensers, and subsequently assembling them in an amplifier, is so great as to be almost prohibitive in the manufacture of commercial radio receivers. Thisinventiom'by providing an extremely sharply tuned first stage of highfre uency amplification individually controlled, followed by several stages of tuned high-frequency amplification, all of which are tuned more broadly than the first, and adjustable together by a single control, re sults in a radio receiver having good selectivity with but two tuning controls, regardless of the-number of tuned amplification stages; Another disadvantage formerly introduced by employing a single shaft for the tuning condensers is that this common metal shaft normally forms a' closed loop with the shaft bearings and metal shield of the amplifier as a whole which frquently caused oscillations due to stray couplings with some other closed circuit of the amplifier, such as a loop antenna circuit. The present inven-. tion likewise overcomes this disadvantage.

Referring again to Figs, 1 and 2, the dotted'lines indicated at 5 represent the vertical walls of the box shields separating and electromagnetically isolatingthe several stages. The voltmeter shown upon the front panel of the receiver, and here included within compartment 3, may be so connected that it indicates at will the voltage of either the lowor the high-tension current supplied to the receiver. 'Rheostats R and R, upon the control panel 24, are adjusted by means of knobs 22 and 23, respectively, to control the filament current of the vacuum tubes. Rheostat R is arranged to control the filament current of the high-frequency amplifyingtubes simultaneously, while rheostat- R is ar-' ranged to control the filament current of the detector tube V, alone. Because of the fact that vacuum tube detectors function most effcctively with a certain critical filament .currentit is desirable that the detector filament be independently controlled. On the other hand, high fr'equency amplifying tubes are not critical-as to their filament tem erature way affecting the tuning adjustment. It is sometimes desirable, however, that the filament of but one of the high-frequency amplifret tubes be separately a justable for volume control. This complete control of the highfrequency energy ismade possible by the present invention through the provision of substantiallystotal capacitive and electromagnetic isolation of the separate stages, ex-

cept for the desired interstage coupling; the

' quency stage effects an absolute controlfrom zero to maximum of the energy passed on to a succeeding stage, without affecting the tuning adjustments of the amplifier, and

without introducing signal distortiom Fig. 3 is a circuit diagram of the high-freuency amplifying and detector portion of t 0 receiver herein illustrated and described.

'Thelow-frequency amplifier shown in compartment (5), Fig.2, has been omitted from the diagram of Fig. 3 inasmuch as the lowfrequency amplifier is not an essential fea ture of t i that any suitable low-frequency amplifier may be connected to the terminals marked Output in Fig. 3; and that a suitable indicating device" such as a loud speaker may be 'connected'to the low-frequenc terminals marked Output in Fig. 2.- ig'.v3 illustrates several features of the present invention which are best shown by means of a circuit diagram, and which, for simplicity, were omitted from the preceding figures. ,It will be noted that the high-frequency stages (1), (2) and (3) include neutralizing coils 7, 10 and 13, respectively, one such 0011 being connected in series with a capacitybetween the grid electrode and the filament system of each high-frequency amplifyin tube in accordance with the invention dgscribed in Hazeltine Patents Nos. 1,489,228 and 1,533,858. As has been previouslv suggested, the provision of a plurality of efficient high-frequency sta es introduces so strong a'tendency toward un esirable oscillation, due to the high degree of am lification, that extreme precautions must e taken to avoid interstagecouplin s. For convenience, it is customary toemp 0y :1 single current source of low volgage for the several tube filaments, and a single current source of high voltage for the plate electrodes of the several tubes. The five binding posts marked (C), +C),.( AB), (+8) and are intended to be connected to appropriate yoltage sources. The 90- volt connection supplies the late c rcuits of the amplifier. stages and of t egdetector, and the 6-volt connection suppliwgthe filament voltage for all tubes, the negati'vfi'terminals e present invention. It is assumed of the sources of both voltages bein connected together on the (AB) bin ing post. The posts marked (C) and (+C) are intended to be connected in series with the return leads from the grids to the filament s sterns of both low-frequency amplifier tufies V, and V,,, of Fig. 2, in the customary'manner, so as to place a suitable negative voltage upon the grids of those tubes. The (O), binding post is shown not connected in Fig. 3, because compartment (5) has been omitted from that figure. I

Such common source of potential, whether it be batteries, genera-tors, or other source, introduces an impedance common to all of the stages to which it is connected. This impedance is'frequently sufiicient, in the case of a multistage highly efiicient amplifier such as that herein described, to cause undesired oscillations. It will be notedthat the present invention provides for complete electromagnetic and electrostatic isolation of each 'high-frequency'stage; and it appears that when three or more high-frequency stages are employed, all of the means provided by this invention are usually required to prevent oscillations. By completely ,enclosin each individual high-frequenc stage, inc udiirg its vacuum tube, within a x shield, substantially all of the undesired interstage electromagnetic and stray capacitive coupling is eliminatedi The capacitive coupling between the input circuit and the output circuit of each vacuum tube is neutralized as described in the Hazeltine patentsreferred to. A bypass condenser C, is provided in each stage connected directly from the low-potential terminal of the primary windin of each high-frequency transformer T and T directly to; the filament of each tube, respective1y,in order to provide a low-impedance high-frequency return path from the plate electrode to the filament of each tube. A non-inductive resistance R R and R is connected respectively in the plate'or output circuit of each high-frequency amplifying tube between the plate inductance and the plate-circuit battery to force the plate-circuit currents to return to the filament through the by-pass condensers C C C and C system may be at ground potential with respect to all portions of the high-fre uency circuits, a high-frequency choke coil, 1 L

L and L is inserted in series with the ungrounded lead for the filament current supplied to the respective tubes. Any high-fre- In order'that the entire filament quency potentials which tend to build u on I either side of these choke coils are reduced by means of bypass condensers C (3, C 2

and C together with 16 and 17. By bonding all of the box "shields with lowresi'stance electrical connections, and by connecting one terminal of the filament of each vacuum tube to the grounded shield, the entire filament system is reduced, at high frequencies, to ground potential.

In completely-shielded receivers employing a common metal shaft to operate the several tuning condensers, there may be produced harmful stray currents which serve to couple non-adjacent stages. This effect may be eliminated either by dividing the shaft into insulated sections, or by grounding the shaft electrically at the several points 52 (Fig. 2) where it passes through the walls of the boxshields. These ground connections serve to short circuit in several places the electrical loop which otherwise would be formed by the metal condenser shaft, the end bearings and the shield as a whole.

The detector, or rectifyingfube, V is connected in the usual manner. Compartment (4;), in which this tube is located, includes in addition, a non-inductive resistance connected in series with the detector plate electrode. This resistance is inserted primarily to prevent high-frequency currents from flowing through the batteries, but it also incidentally reduces the 90-volt supply to a lower voltage suitable for the output circuit of the detector tube.

By-pass conderisers, C and C are connected, between the grounded shield and one side, respectively, of resistance R to assist in preventing high-frequency current from flowing through the detector output circuit.

It will be noted that a tuning condenser, C C and C is connected across the terminals of the input inductance, T 8 and 11, connected between the grid and the filament of each of the high-frequency amplifying tubes. In like manner the input circuit including inductance 1 1 of detector tube V, is tuned by means of condenser C The customary grid condenser C and grid leak R are connected in the grid circuit of the detector tube in order to maintain the grid electrode at a suitable operating potential with respect to the filament. Each of the tuning condensers after the first may be connected in parallel with a small adjustable condenser,

indicated, respectively, in Fig. 3 by reference characters C C and C The purpose of these auxiliary condensers is to determine the minimum capacity value of the tuning condensers which are adjusted by means of the common shaft. Small manufacturing variations in the tuning-condensers, in the inductance coils and in the Wiring may thus be compensated once for all. A given ampli- V fier or radio receiver, after assembly. should be so adjusted by means of these small con.- densers that the component uni-controlled l 130 circuits are 1n resonance at the high-freuency end of the operating frequency stage. If the inductance of all of the coils is substantially the same, this condition will be maintained over the entire'range, inasmuch of the amplifier. In order to obviate this unstable condition" recourse has been made to the invention of W. A. MacDonald, for which a United States patent application, Serial No. 85,615, was filed February 3, 1926. In accordance with that invention, the grid connection of the first high-frequency amplification stage is tapped on to the input inductance of that stage at a. suitable point, usually found to be near the electrical center of the inductance. Such an arrangement is illustrated in Fig. 3 where the grid connection of the first amplifying tube V is represented as being connected to the center of inductance coil.T,. The tuning condenser C is connected across the extreme terminals of inductance T and is employed to sharply tunethe first stage input circuit independently of the succeeding circuits of the amplifier. One of the important factors contributing to instability in highly efficient amplifiers is the commercial variation in. the plate grid capacity of vacuum tubes. I According to the MacDonald invention referred to, the input circuit impedance between the grid and the filament is reduced to a value sufficiently small to prevent certain detrimental voltages due to the mentioned unavoidable couplings from being built up in the grid circuit. For the same reason this expedient provides a satisfactory tube-capacity tolerance. While it is usually desirable to connect the antenna, if it be of the loop type, or of the open tyfpe of short length, directly to the grid of the rst tube, as represented in Fig. 3, it may, of course, be coupled condenser C connected across its terminals.

In that event the inductive loop T would be operated in the customary manner outside the box shield 1), and, of course, the antenna A, and in most cases the ground G, would be disconnected.

The resonance curves shown in Fig. 4 are.

representative of the-sharpness of tuning (a) in the first stage, and (b) in the successive stages of high-frequency amplification. It

should be understood that these curves are given merely by way of exaimple to illustrate graphicall the source of certain of the features of t is invention, which are: first, a sharply tuned first stage separately tunable by a single control; and-second, alurality of subsequent stages simultaneousi; tuned broadly with respect to the first stage, by a second single control. Curve A in Fig. 4 shows graphically the tuning characteristics of the first high-frequency amplification stage of a receiver designed and built accordmg to the provisions of the present invention. This curve indicates that the first stage is tuned as sharply as ing the quality 0 the received si through cutting of the side bands. t is now generally agreed by those skilled in the art, that the amplification of the received oscillations must not be reduced more than fifty percent (50% within a frequency band eight thousand (8000) cycles wide, or four thousand (4000) cycles each side of the resonance peak, if satisfactory audible reproduction of the receiver signal energy is desired. Resonance curve A of the first stage should, therefore, be understood to represent the tuning characteristics of a stage designed to have a ve sharp resonance characteristic, but with e foregoing limitation in mind. Obviously, it. could be so designed as to, have any other desired characteristics obtainable within the known engineering limitations of the art. The sharp tuning characteristic may be obtained by employin in the first stage apparatus which is very e cient at high frequencies, i. e. so that the high-frequency resistance is extremely low. An efiicient loop antenna, connected directly in the first stage input circuit, or a short antenna of the open ty loosely coupled to T tuned with an eflizient condenser and a low-resistance coil will, in general, result in a resonance curve at least as sharp as that shown at A. Specifications for a suitable coil are given hereinafter.

Curve B of Fig. 4 represents graphically V V the resonance curve of each and any of the high-frequenc tubeoltisplainthatthefirststage amplification stages subsequent to the These subsequent stages may, within allowable manufacturing tolerances, be identical with each other because each is preceded and followed by a vaculum 9 e trically, isinherently different from the others because it is not preceded by a vacuum tube. As previously explained, it is impracticable to manufacture the tuning condensers and coils sufiiciently alike to enable each 1 of these stages to have a resonance curve as as that indicated by reference characshaizp V r ter .A. The inductance and "capacity elesible without i'mpair-' als ments of these stages are therefore purposely desi ned to tune substantially broader than the first stage. A distinct advantage to be derived from this characteristic is that the fidelity, or quality of signal reproduction, is

then unimpaired, because the admitted side four thousand (64,000) cycles wide, on each side of the resonance peak, at fifty per cent of maximum amplification. The resultant resonance curve of the entire mult istage receiver is the product of the resonance curves of all of the stages. This-resultant curve may, therefore have the same general characterlstics as theresonance curveaof receivcrs employing fcwer stages of increased sharpness per stage. Another pronounced advantage introduced by the broad tuning. is

that the high-frequency transformers and tuning condensers may be manufactured within readily obtained commercial tolerances, and at the same time be capable of satisfactory operation with uni-controlled condenser adjustments.

The curves shown in Fig. 5 represent the amplification per stage at resonance, (a) of atypical transformer as heretofore employed inhigh-frequency amplifiers including vacuum tubes; and (b of a transformer, in accordance with the present invention, em-

ployed in combination with a-vacuum tube,

as shown in Fig. 3. It is apparent from curve B that, since the losses purposely in troduced are a function of the frequency, the

I bands of. a single stage may be at least sixty-j' transformers introduce the very undesirable feature of a large difference in voltage am-' plification between the upper and lower limits of the operating frequency range as shown in curve A. Furthermore, the extremely high degree of 'amplificationobtained at the low waie -length end of the range introduced a great tendency toward undesired oscillations. 1h order to lessen this tendency, it was the practice to reduce the num-.

ber of primary turns of the transformer, the net result being a lower average amplification. On-the other hand, the resent in-. vention allows of a comparatively arge number of primary turns'whereby an increased average amplification is attained. a

The improved amplification characteristic I Y and the desired broadness of tuning are realized injthis invention'through the introduction of capacity and dielectric losseswhich appear in the interstage high-fie quency transformers such asthose repre-. sented in 3 by reference characters T T and T These lossesresult from utilizing a small form upon which to wind the wire, as, for example, a cylindrical insulating tube two inches in diameter; by using wire of rather small gauge, for example No. 30, American wire gauge, sothat the turns are close together; by winding together the turns comprising the neutralizing coil represented in Fig. 3 as 7 10 and 18, respectively, and the turns of the corresponding primary coils 6,

9 and 12, respectively, on the same form so that the turns of the neutralizing coil and the primary of the same transformer lie alternately and adjacent on the form; and by employing-an unusually large number of primary turns, as represented at 6, 9 and 12, respectively. It has been discovered that the mentioned dielectric loss may be regulated ment described herein, the secondary windings 8, 10 and 14 of the transformers T T and T, were each wound on a form 2 inches in diameter, the secondary winding of each transformer being mounted with its form surrounding the form supporting the primary and neutralizing windings thereof.

- At least five objects are attained by this construction: first, more nearly complete neutralization of the combined inter-electrode and between-the-leads capacity is realized by the extremely close coupling between the two coils in question; second, high amplification, or gain, per stage, resulting from the large number of primary turns; third," the introduced capacity and dielectric losses result in the desired broad and easy tuning; fourth, since these losses are effective as resistance inthe coupling transformers and vary in proportion to the inductance-tocapaci'ty ratio as the tuning capacity is varied, the degree of amplification is reduced at the higher frequencies, and consequently the amplification characteristic is nearly uniform over the entire operating frequency range of the high-frequency system; andfifth, the coils being physically small, allow of the use of a containing cabinet of convenient proportions- The arrangement of the common condenser shaft parallel to the control panel also assists in allowing of a conveniently proportioned cabinet. It is believed to be obvious-that many of the improvements introduced bythis invention may with good effect be embodied in transmitting, or in other apparatus, in which a multistage high-frequency amplifier is included- The constants of the principal components of the radio receiver herein illustrated and T T T,-primary 20 turns, #30 A. W.

G. double silk covered wire on 2 inch cylinder; secondary 63 turns #30 A. W. G.

double silk covered'wire on 2 inch cylin dcr; neutralizing windmg 20 turns, #30

A. \V. G. double silk covered wire woundadjacent and parallel to the primary turns.

C,,' ,50O micro-microfarads, maximum.

,, ,15 micro-microfarads, maximum.

C -15 micro-microfarads, maximum.

(1,2,3, ,1 microfarad.

C 1 microfarad.

C 3000 micro-microfarads'.

C 250 micro-microfarads.

16, 17-1 microfarad.

It -40,000 ohms.

R 2 megohms.

L, 100 microhenries, resistance.

I claim:

1. In an amplifier, a plurality of amplifying stages; eachstage includin at least one high-frequency winding, a variable con denser and a vacuum tube; an input circuit for each vacuum tube includin one of said windings tuned by one of sai conden sers; an output circuit for each vacuum tube including a winding coupled to a succeeding vacand 1 ohm C.

winding, said variable condenser an said vacuum tube of each stage; a conductin metal shaft extending through said meta box-shields for operating several of said variable condensers simultaneously; and short electfical connections between said metal shaft and said box shields substantially at the points where said shaft passes through the walls of said box-shields whereby stray couplings due to said common metal shaft are prevented.

2. In a high-frequency vacuum tube amplifier comprising more than two stages, means for coupling the first stage thereof to an antenna system, said first stage being designed to tune sharply to all frequencies within a predetermined band, individual means for tuning said first stage, means for successively coupling a plurality ofthe remaining stages -m cascade, each of said plurality of remain- I each of said boxsaid first sta e) at allot the frequencies within said pre etermined band, and uni-con.- trolled means for tuning simultaneously all of said pluralit of remaining stages.

3. In a highrequency vacuum tube amplifier comprisin more than two s'ta es, means for coupling t e first stage thereo to an antenna system, said first stage being designed to tune sharply to all frequencies within a predetermined band, individual means for tuning said first stage, means for successively couplmg a plurality of the remaining stages in cascade, each of said plurality of remaining stages being designed to tune broadly ('ascompared with the sharpness of tuning of said first stage) at all of the frequencies within said predetermined band, means for preventing undesirable oscillations in each of said stages whereby a high degree of faithful amplification throughout the frequency range of the amplifier is attained and unicontrolled means for tuning simultaneously each of said lurality of remaining stages.

4. In a ra i0 receiver, a plurality of highfrequency amplifying stages each including a vacuum tube together with tunable coupling apparatus, metal box-shields each enclosing a vacuum tube and associated ap aratus comprising one of said stages, sal box-shields each bein effective to shield the apparatus enclosed t ierein a ainst interstage coupling, ields having an aperture therein large enough to permit the insertion and removal of a vacuum tube, and metal caps each adapted to. form a closure for one of said apertures, said metal caps being designed to be removably secured to their respective box-shields and, when in position,

closing a vacuum tube and associated a paratus con'iprising one of said stages, said 0):- shiel'ds each being effective to shield the apparatus enclosed therein against 'interstzige coupling, each of said bok-shields having an aperture therein large enough to permit the insertion and removal of a vacuum tube, the arrangement being such that when said vacuum tube is in its operating position the free end portion thereof projects through said aperture. and a metal cap for each of said box-shields, each of said metal caps being adapted to be frictionally secured to its box-shield in such a way as to be readily removable andwhen in place to form a complete closure for said aperture whereby to complete the shielding of the apparatus enclosed within the box shield.

In testimony whereof I aflix my signature.

JOHN F. DREYER, JR. 

